1. Field of the Invention
The present invention relates to a gas-insulated switchgear in which a vacuum circuit breaker, a vacuum switch, or the like, is housed in a gas tank having a bellows.
2. Discussion of Related Art
FIG. 4 is a sectional view illustrating a conventional gas-insulated switchgear. In a vacuum switch 1, a main contact assembly constituted by a fixed contact element 3 and a movable contact element 4 is housed in a vacuum tank 2 constituted by an insulation tank 2A and vacuum-sealing metal fittings 2B and 2C. One end of each of fixed terminal 5 and movable terminal 6 is fixedly attached to the fixed and movable contact elements 3 and 4, respectively. In a vacuum sealed state, the other end of the fixed terminal 5 is fixed and the other end of the movable terminal 6 is movably led out to the vacuum tank 2. The vacuum switch 1 is housed in a gas tank 7 filled with an SF.sub.6 gas having gas pressure not less than the atmospheric pressure, and the fixed terminal 5 is electrically and conductively connected to a penetrating bushing 9A through a main circuit conductor 8. The movable terminal 6 is electrically and conductively connected to another penetrating bushing 9B through a flexible lead 10 and a main circuit conductor 11. The movable terminal 6 is further coupled through an insulating rod 12 with a transmission mechanism 13 for transmitting a driving force for causing the movable contact element 4 to perform switching in the axial direction. This transmission mechanism 13 is constituted by a driving rod 14 fixed to the insulating rod 12, a lever 15 coupled with the driving rod 14, and a rotary shaft 16 fixed to one end of the lever 15. The rotary shaft 16 is led out rotatably from the gas-sealed state of the gas tank 7 into the external environment and coupled with a not-shown driving means, for example, an air cylinder or the like.
In the thus arranged gas-insulated switchgear, the vacuum switch as a circuit breaker or a switch is so small that it is possible to make the configuration of the whole apparatus compact. The penetrating bushings 9A and 9B serve to lead currents of the respective main circuit conductors 8 and 11 into another gas tank or into a cable room. Although the sectional view of FIG. 4 shows the configuration of only one phase of a main circuit, usually, a switchgear has a three-phase configuration, in which case, the respective main circuits for the three phases are housed in the gas tank 7. Another words, the main circuits for the other two phases, each of which has the same configuration as shown in FIG. 4, are provided side by side at an interphase insulating distance in the direction perpendicular to the drawing.
The structure for leading out the movable terminal 6 of the vacuum switch from the vacuum tank 2 in the state of being vacuum-sealed is constituted by a cornice-shaped metal vacuum-sealing bellows 17 capable of expansion and contraction in its axial direction. The bellows 17 is disposed in the outer circumference of the movable terminal 6 coaxially therewith in a manner so that one end thereof is hermetically welded to the movable terminal 6 and the other end thereof is hermetically welded to an opening portion 19 of the metal fitting 2C of the vacuum tank 2. By this vacuum-sealing bellows 17, it is possible to absorb the axial-direction movement of the movable terminal 6 while keeping the vacuum state at the time of switching the main contact.
FIG. 5 is a sectional view illustrating a gas sealing configuration portion along the line 5--5 in FIG. 4, in which the rotary shaft 16 is provided through a cushion ring 22, a gas-sealing ring 23 and a bearing 24 in a metal cylinder 21 welded to the gas tank 7. One end 16A of the rotary shaft 16 is fixed to one end of the lever 15 in the gas while the other end 16B of the same is coupled with a not-shown driving means in the external environment. The one end 16A of the rotary shaft 16 is further extended to the left in FIG. 5 so as to be fixed also to the respective ends of levers of the other two phases. Thus, if rotation torque is applied to the rotary shaft 16 by the driving means, the lever 15 changes its angle simultaneously with the respective levers of the other phases. In this case, lever 15 changes its position to the position of lever 15A illustrated by the two-dotted chain line in FIG. 4 to thereby move the movable contact element 4 to switch through the driving rod 14 simultaneously with the corresponding parts of the other phases.
In FIG. 5, the bearing 24 makes the rotary shaft 16 rotatable. The gas-sealing ring 23 is constituted by a ring-shaped rubber material and includes a ring spring 23a for clamping therein the rotary shaft 16 in its radial direction, so that the surface of the rotating rotary shaft 16 is made to contact with the inner surface of the hollow cylinder 21 for the gas-sealing. The cushion ring 22 is made, for example, by fluorine resin, so as to serve as a steady brace in its radial direction. Usually, the inside pressure of the gas tank 7 is about several atms higher than the atmospheric pressure, while the inside of the vacuum tank 2 must be kept at a high vacuum, so that the gas-sealing of the gas tank 7 is not as difficult as the vacuum-sealing of the vacuum tank 2, and therefore, such a rubber packing system as mentioned above is employed in this sealing portion.
In such a gas-insulated switchgear as mentioned above, however, since a vacuum-sealing bellows is caused to expand/contract to the vacuum switch side by the gas pressure in the gas tank, there has been a problem in that the expansion/contraction force is applied to the driving means as a resistant force against the main contact opening operation.
The SF.sub.6 gas in the gas tank is sealed with gas pressure usually higher than the atmospheric pressure, and the gas pressure is applied to the vacuum switch. Accordingly, the vacuum-sealing bellows is caused to expand in its axial direction toward the inside of the vacuum switch, so that a force is exerted to cause the movable contact element to press the fixed contact element through the movable terminal. Therefore, a force necessary to open the main contact becomes much larger as the gas pressure of the gas tank is higher, so that the driving means having a large driving force enough to open the vacuum switch is necessary while taking into consideration and compensating for the application of a resistant force caused by the gas pressure of the gas tank. In addition, in the case of an occurrence of a short-circuit fault inside the gas tank, the gas pressure of the gas tank rises temporarily by arc discharge, and in such a case, the resistant force further increases, so that there exists a possibility that the breaking speed of the vacuum switch decreases or the breaking becomes impossible. The increase in value of the gas pressure due to the short-circuit fault is unstable, such that the characteristic of an opening operation of the vacuum switch becomes scattered and unpredictable.